Herald of War

Mana cost: {3}{W}{W}
Type: Creature — Angel
Power / toughness: 3 / 3
Set: Avacyn Restored #24 (rare)
Price: $3.25 USD

Flying
Whenever this creature attacks, put a +1/+1 counter on it.
Angel spells and Human spells you cast cost {1} less to cast for each +1/+1 counter on this creature.

Herald of War has 4 printings, first in Avacyn Restored (2012). EDHREC ranks it #2,973 among Commander cards. It appears most in Giada, Font of Hope decks (30,070 on EDHREC). 2 public decks on TCG run it. It has 5 official rulings.

Legality

commander: legal
legacy: legal
vintage: legal
pauper: not legal
modern: legal
pioneer: not legal
standard: not legal
brawl: legal

Public decks that run Herald of War

Calling All Angels - Foundations Commander, led by Giada, Font of Hope
Coven Counters - Midnight Hunt Commander, led by Leinore, Autumn Sovereign

Commanders that run Herald of War most

Deck counts from EDHREC.

Often played with Herald of War

Cards that show up in the same decks on EDHREC.

Rulings

2012-05-01: The cost reduction is based on the total number of +1/+1 counters on Herald of War, not just the ones put on it by its own ability.
2012-05-01: If there are additional costs to cast a creature spell, such as a kicker cost, apply those increases before applying cost reductions.
2012-05-01: To determine the total cost of a spell, start with the mana cost or alternative cost (such as a flashback cost) you're paying, add any cost increases, then apply any cost reductions. The mana value of the spell is determined only by its mana cost, no matter what the total cost to cast the spell was.
2012-05-01: The +1/+1 counter is put on Herald of War before blockers are declared and combat damage is dealt.
2012-05-01: A creature spell that's both an Angel and a Human will cost {1} less to cast for each +1/+1 counter on Herald of War.